During the winter season such as −10 degree C. or lower, while such cool outside air undergoes heat exchange in a heat exchanger of this kind of ventilators, exhaust air undergoes heat recovery during passing through an air channel separated from that of the cool outside air. Moisture in the exhaust air is thus frozen and clogs in the heat exchanger at the exhausting channel. It is acknowledged that this clog substantially reduces the exhaust air flow. Reduction of the exhaust air flow causes insufficient supply of heat energy to the outside fresh air to be drawn inside, so that the outside air cannot be warmed up to the room temperature. To overcome this inconvenience, i.e. a resident feels the supplied air rather cool, a method is proposed and disclosed in, e.g. Japanese Utility Model Publication No. H02-103640.
A conventional heat exchange type ventilator is described hereinafter with reference to FIG. 8. As shown in FIG. 8, box-like enclosure 101 comprises the following elements:                inside air inlet 103 and inside air outlet 104 both disposed at the front of enclosure 101 and communicating with room 102; and        outside air inlet 106 and outside air outlet 107 both disposed at the rear of enclosure 101 and communicating with the outside 105. Exhaust air channel A-A′ and supply air channel B-B′ are formed inside of box-like enclosure 101. Exhaust air channel A-A′ connects inside air inlet 103 to outside air outlet 107, and includes exhaust-air fan 108 fixed to a first shaft of motor 109. Supply air channel B-B′ connects outside air inlet 106 to inside air outlet 104, and includes supply-air fan 110 fixed to a second shaft of motor 109. Partition 111 separates channel A-A′ from channel B-B′ and yet crosses the two channels in part. Heat exchanger 112 is placed at the intersection of the two channels. Damper 113 working at around 0 (zero) degree C. includes shaft 114 and hinge 115, so that it rotates, bends, and elongates around shaft 114 and hinge 115. Damper 113 thus can open the supply air channel or close it in part.        